Internet Connection Failed - DNS Problem?

I have Mepis 6.5 running and using dhcp from a server on my home network. It has been working fine for ages, but tonight I was trying to access the internet with firefox and could not get a page to open for any site. I could ping computers on my local network, I had an ip address, correct default gateway and Mepis network assistant reported the correct DNS servers (ISP provided DNS). I did an packet capture with ethreal and it looked like the primary DNS server was being queried by my computer when I opened firefox and the DNS server appeared to respond. I also tried the LiveCD just for laughs and had the same results.

I suspected some kind of DNS problem so I rebooted from hard disk and set up the network to use static DNS. This still did not work, so I reversed the primary and secondary DNS addresses and then I was able to access the web with firefox. This all seems quite odd because I have multiple Windows, Solaris, Mac and Linux systems and they were all working fine while my Mepis system struggled. To add another oddity, while I could not access the internet with firefox, konqueror or synaptic I was able to open up streamtuner and listen to an internet radio stream just fine - curious.

Any comments on what might have happened? I was thinking Mepis did not recognise that it needed to use the secondary DNS server, but then if i was reading the packet capture right then my system thought the primary DNS was responding so there was no reason to try the secondary DNS.